In recent years, as vehicular operation has become quieter, noise arising from tires is an increasingly marked factor contributing to vehicular noise, and there is a growing demand for the reduction of it. Such tire noise is caused by a resonance phenomenon in the tube-shaped cavity formed between the tire and the road surface when a circumferential groove formed in the tire tread surface makes contact with the ground. Generally called air column resonance, this resonance occurs at a wavelength twice the length of an air column, namely, the length of the circumferential groove enclosed in the contact patch of a tire. Conventional methods for reducing this air column resonance are known to be based on the reduction of the number of circumferential grooves or the volume thereof.
Also, methods proposed for tires having circumferential grooves and unilaterally open-ended grooves, which open at one end into a circumferential groove and terminate at the other end within a land portion, are based on reducing the air column resonance through the scattering of it by changing the length and size of the unilaterally open-ended grooves or providing two kinds of unilaterally open-ended grooves having different volumes (see References 1 and 2, for instance).    Reference 1: Japanese Unexamined Patent Application Publication No. 61-125902    Reference 2: Japanese Unexamined Patent Application Publication No. 2007-45324